Book Review

The Big Book of Concepts by Gregory L. Murphy
The MIT Press, 2002; ISBN: 0262134098.

I am ambivalent about this book. Thus I would like to take
the somewhat unusual step of quoting from someone else's review of it. Paul
Bloom is a Yale developmental psycholinguist most likely known to evolutionary
psychologists as Steven Pinker's co-author for "Natural Language and
Natural Selection," the article that put language origins back on the
intellectual map. Writing in Nature Bloom (2003) says that

Murphy . . . reviews a complicated literature with
honesty, clarity and wit. This is going to be the classic text in the field
for a very long time. It is one of those rare cases in which the standard
back-of-the-book blurb is actually true. Anyone seriously interested in
concepts and categorization — seasoned researchers, graduates and advanced
undergraduates, or scholars who simply want to get a sense of the field —
must read this book.

I have no reason to contest that judgment; this is a clear
and thoroughgoing review of a large literature. Bloom then goes on to say:

If you skip to the end of the book looking for a clear
resolution, you will be disappointed. Concepts, Murphy cheerfully concludes,
are a mess. With the exception of the classical view ("a total
flop"), each of the existing theories is the best explanation of a
particular set of empirical findings.

I have no reason to contest this, either. Most investigators
in this field, so Murphy asserts, focus only on a limited set of issues and
propose models that account for those, and only those, issues. None of those
models work when one attempts to account for a wide variety of phenomena.

The "classical view" that Bloom mentions is the
starting point for Murphy's discussion. This view dates back to Aristotle and
holds that concepts are given meaning by necessary and sufficient conditions.
One problem with this view is that it has proven very difficult to specify those
conditions for both real and artificial concepts—a discussion in which
Wittgenstein has been very influential. Another problem is that the view doesn't
admit of gradations. Something either is, or is not, an example of a particular
concept. This is difficult to square with the line of experimental investigation
initiated by Eleanor Rosch and her colleagues in the mid-70s. They have found
that, for example, a robin is more birdlike than a chicken and that a chair is
more furniturelike than a piano. The classical view has no way of accounting for
this well-replicated empirical result.

For these and other reasons the classical view has fallen out
of favor with experimentalists. It has been replaced by accounts that fall in
one of three broad categories: 1) exemplars, 2) prototypes, and 3) theories. On
the first view, conceptual categories are built up by remembering examples as we
encounter objects. New objects are categorized in accordance with the examples
they most resemble. Prototype views posit a summary account for each category,
often in the form of a weighted feature list. When a new object is encounted,
its features are noted and compared with the feature lists for each category. It
is classified in the category with which it shares most features.

The theory view is a bit different. It holds that concepts
exist in rich and structured relationships with other concepts and these
relationships affect concept use. These conceptual complexs are sometimes
thought to constitute intuitive theories. In the domain of folk biology, for
example, Murphy describes (p. 165) how Rips

created a scenario in which an object or animal went
thorugh a transformation. In particular, he described to subjects something
that was much like a bird. This animal, however, had the misfortune to live
next to a toxic waste dump, where chemicals caused it to lose its feathers
and develop transparent, thin wings. It also grew an outer shell and more
legs. At the end of this metaphorphosis, the creature appeared rather
reminiscent of an insect. Rips asked subjects whether the animal was a bird
or an insect and whether it was more similar to birds or insects. Subjects
claimed that this stimulus was more similar to an insect but that it was
still a more likely to be a bird. That is, subjects felt that the
change in outer appearance and even body parts did not constitute identity,
but that the animal's inherent biological properties determined what it was.

The question of just how such folk biology originates,
however, is beyond the scope of this literature.

Murphy reviews a great deal of empirical work on these three
types of theory, including experiments designed to distinguish between one type
of theory and another. Murphy also has chapters on taxonomy, induction, the
development of concepts in infants and children, word meaning, and conceptual
combination. When all is discussed and reviewed Murphy concludes, as Bloom has
noted, that none of the three main approaches—exemplars, prototypes, theory—can
be ruled out on empirical grounds, nor can any of them account for all the
evidence.

In fact, he goes beyond that, to suggest that we will
probably end up with a "mixed" account of concepts that embraces
models of each type:

On the proposal I am making, people attempt to form
prototypes as part of a larger knowledge structure when they learn concepts.
But at the same time, they remember exemplars and these memories may
influence them in a variety of ways. In short, concepts are a mess. (p. 492)

From there Murphy works his way to the comforting observation
that "the diverse nature of conceptual behavior is itself an empirical
discovery" (p. 494).

I am willing to accept this. If then, I have no reason to
dispute this, why am I a bit uneasy about this book?

Actually, it is not so much the book I'm uneasy about, as the
field it reviews. It's not clear that you can get there from here.
What do I mean?

During the 1970s, when cognitive science was beginning to
crystallize as a recognizeable interchange of work in linguistics, philosophy,
psychology, and computer science, a great deal of work was done in the computer-modelling
of human knowledge. This work, in fact, was one impetus for some of the
theory-oriented reseach Murphy has reviewed. But the question of whether or not
concepts were embedded in a rich structure, such as intuitive theories or a
highly structured associated network, was not itself in question. The need for
rich conceptual structures was taken as a starting point. But a major point of
contention concerned just what kinds of concepts (e.g. properties, entities,
events, plans, scripts, frames, etc.) and relations (e.g. is-a, agent, patient,
cause, purpose, before, etc.) were needed to model various domains. Those issues
are still very much alive, but they are not addressed by the research Murphy
reviews, nor is it obvious to me how those techniques could address such issues.

Much of the work on these issues is being done in the name of
knowledge representation (cf. Sowa 2001). But knowledge representation is most
often treated as a matter of logic or of software engineering without any
empirical component beyond the question of whether or not a computer program
runs successfully. Of course, getting empirical evidence on how brains organize
these conceptual structures is not easy; it is one of the most difficult issues
facing the human sciences. What makes me uneasy about Murphy's book is not so
much that it does not have much to say about these difficult matters. From an
empirical point of view—and Murphy is certainly an empiricist—there isn't
much to say.

What bothers me, then, is that he has not adequately
acknowledged these issues, that the range of "concepts" under
investigation in this particular literature does not come close to the range of
"concepts" that furnish the human mind. Thus The Big Book of
Concepts appears to be but a prolegomena to something else: the integration
of these models and empirical techniques into a richer intellectual undertaking.
If you are interested in that richer undertaking you probably need to know what
is in this book, but mostly as background. To actually navigate in those other
territories you will need to consult other literatures.